Add negative numbers to math expression

Hmm… weird that wasn’t caught earlier by devs.
I am working on a parser and that was an issue I had to fix very early even though majority of my parser is still incomplete.

The attempts to support unary ops seem to exist in the source code (Editor/BlueprintGraph K2Node_MathExpression), but apparently the current implementation attempts break other functionality. I think it’d be worth looking into, but this file is about 2,800 lines of code, so it might take a while.